This invention relates to an improved process for the manufacture of thianthrene. The compound thianthrene, (C.sub.6 H.sub.4).sub.2 S.sub.2 is useful as a chemical intermediate in the preparation of dyestuffs such as thianthrene vat dyes. Since the first preparation of thianthrene by J. Stenhouse in 1869, various processes for its preparation have been proposed and tried on a laboratory scale, with widely varying results. Little, if any, effort has been directed toward the development of a process suitable for commercial manufacture. Insofar as the applicants have ascertained, only one method described in the prior art for the preparation of thianthrene is of any significance (J.A.C.S. Vol 78, 2163-2164, 1956). In this method, sulfur monochloride is reacted with refluxing benzene in the presence of aluminum chloride, and the reaction product treated with iced hydrochloric acid and steam distilled to remove benzene and steam volatile impurities. The aqueous layer is then decanted off, and the residue filtered. The product is warmed with a sodium hydroxide solution, filtered and washed successively with water and ethanol. The residue is refluxed with an excess of glacial acetic acid, cooled, filtered and washed with ethanol to give 86% yield of crude brown thianthrene. The purity of the product may be improved (with a consequent decrease in yield) by additional distillation and recrystallization from glacial acetic acid.
While this laboratory scale method produces thianthrene, many disadvantages prevent its adoption for large scale commercial production.
Sublimation and subsequent condensation of by-product sulfur is a major problem, leading to plugging of pipelines and creation of pressure buildups which are a detriment to sustained operation. Frequent cleaning of the pipelines and the condensing system is a major industrial problem requiring heavy down time. Reactions utilizing less than 5.0/1.0 stoichiometric proportions of benzene to sulfur monochloride produce a semisolid end product whose rheology is not compatible with proper mixing or gravity discharge of the products from the reactors. This method involves numerous unit operations to make pure thianthrene (no data is indicated in the literature on the percent purity of thianthrene attainable). Additional drawbacks to the adaptation of such a method to commercial production include the use of substantial quantities of ethanol (a federally regulated solvent) and extremely corrosive glacial acetic acid. Furthermore the thianthrene produced remains contaminated and occluded with by-product sulfur and diphenyl sulfide.
It is among the principal objects of this invention to provide a process for the production and recovery of thianthrene and thianthrene based products which is direct and simple and which is adaptable to large scale commercial operation. A further object is to provide a method whereby a high purity grade of thianthrene may be produced in high yields. A still further object is to provide a process for the manufacture of thianthrene which is easily adaptable to subsequent processes such as halogenation for the preparation of thianthrene-based end products. Other objects will become apparent to those skilled in the art on consideration of the complete specification and claims.